The shaping and working of metal frequently leaves a film of oil upon the object produced. This oil on the surface of the metal may deleteriously affect the subsequent use of the item produced. For example, an oily film will likely interfere with and even preclude the adherence of a paint to a metal surface.
As a result, a section of metal, after undergoing various metalworking operations, must have its surface cleaned of organic, oily substances. A process frequently employed to accomplish this object involves simply immersing the metal object into a caustic aqueous solution. Various solutions with different components and additives have performed this function well and effectively removed organic components from metals.
The problem arises, of course, that the organic components removed from the metal remain in the caustic solution as contaminants. After a number of uses, the level of these contaminants will increase to the point where the solution no longer serves its purpose.
Metalworking companies, faced with this problem in the past, have simply disposed of the contaminated caustic solution by dumping it down a drain. This technique of refreshing the cleaning solution faces a number of significant drawbacks. First, of course, the cleaning solution has an inherent value which accompanies the solution down the drain. Furthermore, various governmental authorities no longer tolerate such dumping of potent chemicals into the sewer system. If the sewage enters a body of water without treatment, the chemicals in the cleaning solution will, of course, adversely affect the quality of that water.
More frequently, though, the sewage undergoes treatment before finding its way into a water system. The chemicals in the sewage from the cleaning solution immensely add to the burden of treating that sewage to the desired degree of water purity. Consequently, the unit of government responsible for the particular sewer system involved may totally prohibit the introduction of such potent chemicals into the system. Alternatively, it may impose a severe charge on the dumper in order to compensate for the burden thus imposed. In any event, a company with a contaminated cleaning solution may now find it impropitious or impossible to dump it into the sewer.
Many metal workers, faced with this problem of contaminated and undisposable cleaning solution, have it hauled away by tank truck. This procedure, of course, incurs the economic burden of paying for the removal service, in addition to the necessity of purchasing the components for further cleaning solution. Moreover, once a solution becomes spent, the metal-working company may have to cease this phase of its operation until its truck arrives and removes the useless liquid. Alternatively, the metal company could make the financial investment of developing sufficient suitable storage capacity for the spent solution until the trucker arrives. Either course, though, represents an appreciable expense to the company.
Various techniques exist for removing comparatively nonpolar components from a polar aqueous solution. The simplest of these involves the use of a separatory funnel in conjunction with an organic solvent. Shaking the aqueous solution together with the organic solvent typically results in the organic components moving from the aqueous phase to the organic phase. This method, of course, presumes the use of an organic solvent that forms a separate phase from the aqueous solution.
Clearly, however, this method of removing the organic components has no practical utility for a metalworking shop. To mix the aqueous solution with an organic solvent represents a time-consuming, arduous, and wasteful proposition. This task may also remove the cleaning solution from active use for an unacceptably long period of time. Moreover, even were this technique to find use in this situation, it would simply substitute, for the contaminated cleaning solution, a contaminated organic solvent; the latter would also pose a disposal problem.
A somewhat more sophisticated approach appears in D. McDonald's U.S. Pat. No. 3,365,395. In it, he treates raw sewage in order to produce clean water for release into the receiving lake or river. One apsect of the extensive treatment involves mixing a halogenated hydrocarbon solvent with the body of sewage at several points in order to remove greases, fats, and oils entrained within it. He then distills his solvent to allow its reuse.
The apparatus shown by McDonald apparently works well for sewage. However, it would still suffer the drawbacks discussed above for the simple separatory-funnel procedure if used for metal-cleaning solutions. Again, it would remove the aqueous solution from actual use in cleaning metals in order to purify it of the contaminants. Moreover, the extensive and expensive apparatus shown represents an investment totally out of proportion with the magnitude of the problem posed.
J. G. Miserlis et al., in their U.S. Pat. No. 3,803,005, attempt to avoid the problem of the contaminated aqueous solution by dispensing with it entirely. Instead, they use the hydrocarbon solvent trichloroethylene directly to degrease metals. However, having open vats for dipping metals into trichloroethylene allows uncontrollable amounts of the solvent to escape into the atmosphere. There, it would pose a health hazard to the employees, and, consequently, require extensive ventilating apparatus to remove it from the working atmosphere. Moreover, trichloroethylene will not remove stains.